What Is Agnosia?

Agnosia is the loss of the ability to recognize objects,
faces, voices, or places. It is a rare disorder. People with agnosia can still
think, speak, and interact with the world normally.

Agnosia usually affects only a single information pathway
in the brain. For example, people with visual agnosia won’t be able to name an
object placed in front of them or describe its use. But they will still be able
to reach for it and pick it up. Once they are holding the object, they will be
able to use their tactile information pathway—their sense of touch—to identify
its use.

What Causes Agnosia?

Agnosia occurs when the brain suffers damage along certain pathways.
These pathways connect the primary sensory processing areas to the parts that
store knowledge and information. Primary sensory processing areas include the
visual and auditory cortices.

Agnosia is usually caused by lesions on the parietal and
temporal lobes of the brain. These lobes are where semantic information and
language are stored. Lesions can be created by strokes, head traumas, or encephalitis.
Other conditions that damage or impair the brain can also cause agnosia. These
conditions include dementia, carbon monoxide poisoning, and anoxia.

Types of Agnosia

There are three main types of agnosia: visual, auditory,
and tactile.

Visual agnosia is the most common type of agnosia. This
might be because humans’ visual processing areas are so large and complex (Coslett, 2007).

Visual Agnosia

Visual agnosia occurs when there is brain damage
along the pathways that connect the occipital lobe of the brain with the
parietal and temporal lobes. The occipital lobe assembles incoming visual
information. The parietal and temporal lobes understand the meaning of this information.

Apperceptive
Visual Agnosia

Visual apperceptive agnosia is the difficulty in assembling parts of an image
into an understandable whole. People with the condition may have difficulty
understanding how objects are related to one another. When trying to copy a
picture of a circle, an apperceptive agnosic might draw a series of concentric
scribbles. People with this condition can still use vision to navigate their
environment and pick up objects without trouble. Apperceptive visual agnosia is
usually caused by lesions to the parietal or temporal lobes on both sides of
the brain.

Associative
Visual Agnosia

Associative visual agnosia is the
inability to recall information associated with an object. This can include an
object’s name, use, or origin. Unlike an apperceptive agnosic, an associative
agnosic can copy a picture without trouble. But he or she may not be able to
name the object in the drawing. For example, an associative agnosic who is
shown a padlock will be able to recognize it and use it, but won’t be able to say
what it is.

Prosopagnosia

Prosopagnosia is the inability to recognize faces. It is caused when there
is damage to the fusiform face area (FFA), a highly specific region of the
brain that recognizes faces. Difficulty with facial recognition can also occur
in Alzheimer’s disease because brain deterioration damages this region.

Individuals with autism often
experience difficulty recognizing faces. Autistic people do not use their FFA
to see faces. Instead, each develops his or her own unique neural pattern for
facial recognition (Pierce, et al.,
2001).

Achromatopsia

Achromatopsia is the loss of color vision. It is caused by lesions in the
V4 region of the brain. When a lesion separates V4 from the language areas, the
result is color anomia. This is the inability to name colors despite being able
to perceive them.

Agnosic Alexia (Pure Alexia)

Pure alexia is the inability to recognize words visually. People with pure
alexia cannot read. They can still speak fluently and can usually write without
difficulty.

Akinetopsia

Akinetopsia is the inability to
perceive motion. People with this condition see moving objects as a series of
stills, like an object moving under a strobe light.

Auditory Agnosia

Pure Word
Deafness

Pure word deafness is the inability to repeat or understand spoken
language. It develops when the A1 sound-processing region of the brain is
disconnected from its language centers. People with pure word deafness can
still recognize environmental sounds. They can also still read and write.

Phonagnosia

Phnonagnosia is the inability to recognize and identify familiar voices. It
develops when the brain suffers damage to a certain part of the sound
association region. This region is located in the right half of the brain.
People with this condition can still understand the words being spoken. They
can also recognize environmental sounds or sounds made by objects.

Tactile Agnosia

Astereognosis

Astereognosis is the inability to identify objects by touch. People with
this condition cannot associate information
regarding size, weight, and texture with the relevant words. They can still
name objects by sight. They are also able to draw pictures of objects, as well
as reach for them.

Autotopagnosia

Autotopagnosia is when a person loses the ability to orient the parts of his
or her own body. It is caused by damage to the left parietal lobe of the brain.
Normally, a person has an awareness of his or her limbs are in space at all
times, even with closed eyes. This awareness gets distorted when the brain’s
internal representation of the body is damaged.